1. Field of the Invention
The present invention relates to a method for producing thin film transistors (hereinafter referred to as TFT), in which is used a metal element such as typically nickel capable of promoting the crystallization of silicon.
2. Description of Related Art
Known are TFT having a silicon film formed on a glass or quartz substrate. Most commercially-available products are amorphous silicon TFT having an amorphous silicon film as the active layer.
TFT are essentially used in active matrix circuits in active matrix-type liquid crystal display devices.
TFT having an amorphous silicon film as the active layer are defective in that they could be put into practical use only for N-channel devices, and that their acting speed is very low. (It may be said that TFT are used only in active matrix circuits because of such defects.)
As one technique for solving the problem, known is a method of using a crystalline silicon film as the silicon film constituting the active layer.
To obtain such a crystalline silicon film, employable is any of laser irradiation or heating.
The former method of laser irradiation comprises applying a laser ray to an amorphous silicon film having been formed through CVD or the like to thereby crystallize the film.
The latter heating method comprises heating an amorphous silicon film having been formed through CVD or the like to thereby crystallize the film.
For the laser irradiation method for crystallization, however, commercially-available laser oscillation devices are not still at practicable level, and therefore the method is problematic essentially in point of the oscillation stability. For that reason, the crystalline silicon films to be obtained in the method are problematic in point of their uniformity and producibility.
On the other hand, the heating method may produce films of stable quality. However, as requiring high heating temperatures, the method is problematic in that glass substrates are difficult to use therein. In addition, since the films formed in the method are in a definite polycrystalline condition, they shall have therein grain boundaries which are unstable factors.
Given that situation, one problem is how to lower the heating temperatures for crystallization. Another problem is of attaining a step of producing films without forming any definite grain boundaries therein.
To solve those problems, the present applicant has developed a technique of crystallizing an amorphous silicon film at a temperature lower than that for conventional crystallization. In the technique developed, used is a metal element such as typically nickel element for the crystallization.
Precisely, the technique is based on a method comprising introducing a predetermined metal element such as typically nickel into an amorphous silicon film followed by heating the film to convert it into a crystalline silicon film.
According to this technique, the crystalline silicon film obtained through heat treatment at about 600xc2x0 C. or so to which glass substrates are resistant shall have a high degree of crystallinity.
Regarding the quality of the film to be obtained in the technique, the film does not have any definite grain boundaries therein, and is therefore superior to any other conventional, definite polycrystalline silicon films.
However, the method where a metal element is used is problematic in that the metal element used remains in the crystalline silicon film formed.
The metal element remaining in a semiconductor film acts as a trap center and has some negative influences on the characteristics of TFT comprising the film. Therefore, it is desirable to reduce as much as possible the concentration of the metal element remaining in a semiconductor film that constitutes the active layer of TFT.
The invention disclosed herein is to provide a technique of improving TFT having, as the active layer, a crystalline silicon film to be formed by the use of a metal element capable of promoting the crystallization of silicon, and is characterized in that the concentration of the metal element remaining in the active layer of TFT is reduced as much as possible.
One aspect of the invention disclosed herein is a method for producing a semiconductor device, which comprises;
a step of forming a crystalline silicon film in the presence of a metal element capable of promoting the crystallization of silicon,
a step of applying a solution containing phosphorus, selectively to a part of the surface of the crystalline silicon film, and
a step of heating the film to thereby make the metal element gettered in the region to which the solution has been selectively applied.
In this aspect, nickel is most favorably used as the metal element capable or Promoting the crystallization of silicon. This is because nickel is the most effective for the intended crystallization and gettering. Especially for the gettering, the combination of nickel and phosphorus is the most effective.
The metal element may be one or more selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Au, Ge, Pb, In and Sb.
In the heating step, in general, the film may be heated in any ordinary heating furnace equipped with a heater. Apart from this, also employable is so-called RTA for exposure to strong radiations.
The phosphorus-containing solution may be a solution containing P2O5 and/or HxPO3.
As the phosphorus-containing solution, also employable is a phosphorus-containing, silicon oxide film-forming liquid. As one typical example of the liquid, mentioned is an OCD solution manufactured by Tokyo Chemical Industry Co.
This solution is, after having been applied onto a substrate and baked thereon, solidified to give a silicon oxide film. The solution requires a step of solidifying it to have a solid phase.
As the phosphorus-containing solution, further employable is a solution containing a phosphoric acid compound dissolved therein.
After the phosphorus-containing solution is applied to the crystalline silicon film, the film is then processed according to any of the following methods (1) to (3) to thereby make phosphorus kept in contact with a part of the surface of the crystalline silicon film;
(1) Blowing off an excess of the phosphorus-containing solution by a spin dry process,
(2) Blowing off a liquid component of the phosphorus-containing solution by heating and dry process,
(3) Forming a film such as silicon oxide film by baking to solidify the phosphorus-containing solution.
Then, heating the film makes phosphorus slightly diffuse (bleed) into the crystalline silicon film, by which nickel is gettered.
Another aspect of the invention is a method for producing a semiconductor device, which comprises;
a step of applying a solution containing a metal element capable of promoting the crystallization of silicon, selectively to a part of the surface of an amorphous silicon film to thereby make the solution kept in contact with the selected part,
a step of heating the film so as to diffuse the metal element from the region of said part to another region, thereby attaining crystal growth in the direction parallel to the surface of the film,
a step of applying a solution containing phosphorus, selectively to the region of said part to thereby make the solution kept in contact with said part, and
a step of again heating the film to thereby make the metal element gettered in the region of said part.
Still another aspect of the invention is a method for producing a semiconductor device, which comprises;
a step of applying a solution containing a metal element capable of promoting the crystallization of silicon, selectively to a part of the surface of an amorphous silicon film to thereby make the solution kept in contact with the selected part,
a step of heating the film so as to diffuse the metal element from the region of said part to another region, thereby attaining crystal growth in the direction parallel to the surface of the film,
a step of applying a solution containing phosphorus, selectively to the region of said part to thereby make the solution kept in contact with said part, and
a step of again heating the film to thereby make the metal element move along the route opposite to the route of said diffusion.
Still another aspect of the invention is a method for producing a semiconductor device, which comprises;
a step of forming a crystalline silicon film by the action of a metal element capable of promoting the crystallization of silicon,
a step of making a phosphorus-containing material kept in contact selectively with a part of the surface of the resulting crystalline silicon film, and
a step of heating the film to thereby make the metal element gettered in said part.
In this aspect, the phosphorus-containing material is formed by applying a phosphorus-containing, silicon oxide film-forming liquid, selectively to the intended part of the surface of the crystalline silicon film, followed by heating it to form a silicon oxide film on the part of the surface of the crystalline silicon film.
In one embodiment of this aspect, formed is a PSG film or a silicon oxide film from the Tokyo Chemical""s OCD solution.